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  1. There is a flock of birds in the sky.

  2. There is a flock of birds on the sky.
what are the difference?
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in the sky (the only prep. that sounds correct to me)
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'In' indicates inside of, within, enveloped. The fish are in the ocean. The toy is in the box.

'On' is like attached to, adjacent, touching. The writing is on the wall, The fish is on the stove.

If the birds are 'on' the sky it means there's a surface for them to be connected to, but that isn't the case and also the 'sky' here is more of a place, like the birds are in America. To describe their location we say they're inside the area, the sky.
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zenkg
    1. There is a flock of birds in the sky.
  1. There is a flock of birds on the sky.
what are the difference?
zenkg
    1. There is a flock of birds in the sky.
  1. There is a flock of birds on the sky.
what are the difference?